An organic electroluminescence device (hereinafter the term “electroluminescence” is often abbreviated as “EL”) is a self-emission device utilizing the principle that a fluorescent compound emits light by the recombination energy of holes injected from an anode and electrons injected from a cathode when an electric field is impressed. Since C. W. Tang et al. of Eastman Kodak Co. reported a low-voltage driven organic EL device in the form of a stacked type device (Non-patent Document 1, or the like), studies on organic EL devices wherein organic materials are used as the constituent materials have actively been conducted. Tang et al. used tris(8-hydroxyquinolinol aluminum) (hereinafter referred to as Alq3) for the emitting layer and a triphenyl diamine derivative for the hole-transporting layer. The advantages of the stack structure are to increase injection efficiency of holes to the emitting layer, to increase generation efficiency of excitons generated by recombination by blocking electrons injected in the cathode, to confine the generated excitons in the emitting layer, and so on. Like this example, as the structure of the organic EL device, a two-layered type of a hole-transporting (injecting) layer and an electron-transporting emitting layer, and a three-layered type of a hole-transporting (injecting) layer, an emitting layer and an electron-transporting (injecting) layer are widely known. In such stack structure devices, their device structures and fabrication methods have been contrived to increase recombination efficiency of injected holes and electrons.
As luminescent materials for an organic EL device, a chelate complex such as tris(8-quinolinorate) aluminum complex, and luminescent materials such as a cumarin derivative, a tetraphenyl butadiene derivative, a bistyryl arylene derivative and an oxadiazole derivative has been known. It was reported that those materials emit light in a visible region from blue to red, and realization of a full color display device has been expected (for instance, see Patent Documents 1 and 2). Also, a phosphorescent-type organic EL device using an organic phosphorescent material in addition to the luminescent material in the emitting layer is proposed recently (for instance, see Non-Patent Document 2). In the emitting layer of this phosphorescent-type organic EL device, higher luminous efficiency has been attained by the use of the singlet state and the triplet state which are exited states of the organic phosphorescent material. It is considered that singlet excitons and triplet excitons are generated in a ratio of 1:3 depending upon the difference in the spin multiplicity at the time of recombination of electrons and holes in the phosphorescent-type organic EL device. Therefore, it is considered that if the phosphorescent material is used, the luminous efficiency of 3 to 4 times as high as that of the device using a fluorescent material alone can be attained.
Further, in such a phosphorescent-type organic EL device, the structure in which an anode, an organic emitting layer, a hole blocking layer, an electron-transporting layer and a cathode are stacked in sequential order has been used, in order to maintain the triplet exited state or not to quench triplet excitons. By providing a hole blocking layer between the organic emitting layer and the cathode, which limits the movement of holes out of the organic emitting layer and has an ionization potential (Ip) larger than that of the emitting layer, holes are efficiently accumulated within the emitting layer, probability of recombination with electrons can be increased and higher luminous efficiency can be attained (for instance, see Patent Documents 3 and 4).
It was reported that a well-known phenanthroline derivative (BCP and Bphen, for example) was used for the above hole blocking layer so that the luminous efficiency increases. However, BCP, Bphen and the like have poor oxidation resistance, and there is the necessity of improve on the lifetime of the organic EL device using them. Then, an organic El device using a fused polycyclic aromatic compound having a phenanthrene and adding Ir(ppy)3 to the emitting layer is disclosed, and it is improved in the lifetime at room temperature to some degree (Patent Document 5). However, in view of practical use of the phosphorescent-type organic EL device, it is required to improve the lifetime under a driving condition at high temperatures. It is considered that under a driving condition at high temperatures, the injecting properties and transporting properties of holes and electrons from the electrodes vary from those at room temperature (for instance, Non-patent Document 3), and carrier balance within the device is much different from that at room temperature. Thus, the conditions for constituting a device to maintain a desired carrier balance under a driving condition at high temperatures have not yet been found, thus posing a large problem to obtain an organic EL device having a prolonged lifetime.